In a medical technology field, electronic endoscopes have been widely known and practically used as a device for observing a body cavity of a patient. Examples of an endoscope of this type are described in Japanese Patent Provisional Publications No. HEI 2-293709A (hereafter, referred to as patent document #1), No. HEI 6-308381A (hereafter, referred to as patent document #2), No. HEI 8-122632A (hereafter, referred to as patent document #3), No. 2004-61763A (hereafter, referred to as patent document #4), No. 2004-354888A (hereafter, referred to as patent document #5), and No. 2007-249189A (hereafter, referred to as patent document #6). The endoscope is designed to be thin so as to reduce burden on a patient during insertion into the patient's body cavity. In each of the patent documents #1 to #6, an exit pupil distance of the endoscope optical system is shortened so as to decrease the diameter of the endoscope optical system.